There have been several prior art proposals for multiple injection of liquid plastic. Examples of these are shown in U.S. Pat. No. 3,758,252 as well as application Ser. No. 531,976 filed Dec. 12, 1974 now abandoned and application Ser. No. 459,707 filed Apr. 10, 1974 now U.S. Pat. No. 3,915,610; all commonly owned by applicant. These patents and applications show various devices which in many cases are well suited for multiple injection of plastic into a mold.
However, these prior art devices, while working well in many instances, do require separate complete devices for each separate mold, and do present certain problems of maintaining the plastic hot during the entire molding cycle. Also, these devices represent rather expensive manufacturing techniques thus making the devices rather expensive to manufacture. However, the basic principle of the curved passage multiple injection concept of U.S. Pat. No. 3,758,252 is sound and has proven to be effective.